The proposed studies utilized freshly isolated hepatocytes to evaluate insulin binding and insulin action in the liver of older obese, fasted, and diabetic rats. These studies indicate that with regard to insulin responsive AIB uptake and insulin binding, hepatocytes from older, spontaneously obese rats are similar in all respects to those from younger, lighter animals. Hepatocytes from fasted animals have the same number of insulin rceptors per cell as those from control and older obese animals, yet they are significantly smaller and are insulin resistant. Hepatocytes from streptozotocin diabetic rats have increased numbers of insulin receptors and are more sensitive to insulin. Additional studies are evaluating other insulin responsive parameters such as lipid and protein synthesis, the effects of insulin mimickers which act distal to the insulin receptor, and, in primary cultures of hepatocytes, the effects of agents which alter insulin sensitivity. The relation between the insulin receptor interaction and insulin degradation is also under investigation. In isolated adipocytes, we have evaluated the effects of membrane fluidity on hexose transport, insulin binding, and insulin action (J. Biol. Chem. 254:2619, 1979). These studies indicate that the physical state of membrane lipids can modify hexose uptake and insulin action in adipocytes and that insulin activates hexose uptake by a mechanism other than an increase in the number of available carriers. Additional studies are in progress evaluating agents which alter membrane fluidity and th mechanism of the temperature related decrease in receptor number.